The optical fiber is obtained by proportional hot drawing (fiber drawing), i.e. drawing while maintaining the various different portions of the preform.
Preforms can be obtained by CVD methods such as MCVD or by the vapor axial deposition (VAD) method.
When depositing by MCVD, successive layers of oxide compounds are deposited inside a silica tube referred to as a substrate tube, and they are then vitrified by means of a torch. These layers correspond to the core and to the inner portion of the cladding of the preform. They present a refractive index that varies in compliance with the properties that are desired for the optical fiber.
It is desirable for reasons of keeping down method costs to obtain preforms that are of large capacity. It is also desirable to have optical fibers that are of great length so as to avoid connection losses.
The capacity of a preform is associated with the ratio of the outside radius of the inner cladding to the core radius, also referred to as the b:a ratio. For a given core diameter in the drawn fiber, preform capacity is easier to achieve with small values for the ratio b:a.
The capacity of a preform thus depends on the quantity of core material which can be deposited inside the substrate tube. Unfortunately, when the core is close to the substrate tube, purity requirements for the substrate tube material becomes increasingly strict. It must be of very high purity in order to prevent impurities migrating towards those portions of the fiber that participate in propagating light. Otherwise, propagation properties are observed to be degraded. Thus, it is generally necessary to deposit an “inner” cladding layer inside the substrate tube before depositing the core. However this layer can be very thin if a high purity substrate tube is used.
After the layers corresponding to the core and to the inner cladding have been deposited, the tube is closed up, an operation that is known as “collapsing”. This provides a “primary” preform.
The refractive index of the layers corresponding to the core and to the inner cladding is controlled by varying the concentration of chemical dopants.
Thus, germanium is often used to increase the refractive index. Doping using fluorine derivatives or boron compounds serves to reduce the refractive index.
The refractive index can also be affected by other compounds that are present. Thus, phosphorus which is added for the purpose of improving the optical qualities of the fiber also increases its refractive index a little.
The deposition rate of MCVD is limited by the transfer of heat through the substrate tube. In order to optimize conditions for MCVD, substrate tubes are therefore of relatively small thickness. After the tube has been collapsed into a primary preform, the thickness of the tube is increased by depositing outer cladding.
Two different methods are known in particular for depositing outer cladding. In the first method, the primary preform is inserted into a silica jacket of diameter that is slightly greater, and then a second collapsing operation is performed. This method is also referred to as “jacketing”.
The second method consists in depositing silica on the primary preform from the outside. This additional outer deposition is also known as building up or out. Such outside deposition can be performed by various methods. An advantageous method is plasma-assisted deposition.
At present, a preform obtained by MCVD can be used to obtain about 250 kilometers (km) of optical fiber from a preform that is 1 meter (m) long. In contrast, preforms obtained by VAD make it possible to obtain more than 400 km of optical fiber per meter of preform.
EP-A-0 972 752 discloses a preform of large size obtained by MCVD. However the outer portion of the optical cladding is obtained by the jacketing method. That technique suffers from the drawback of the very high cost of synthetic silica jackets.